1. Field of Invention
The invention is based on a device for measuring at least one parameter of an air flow mass flowing in an air intake line of an internal combustion engine.
2. Description of Prior Art
DE 44 07 209 C2 has disclosed a measuring body that can be inserted into a clean conduit of an intake line of an internal combustion engine to measure the mass of intake air, a so-called air mass sensor, which has a flow conduit that is essentially made up of a measurement conduit, which tapers in the main flow direction, and an adjoining S-shaped deflection conduit. A measuring element is disposed in the tapering measurement conduit. The measuring element can be embodied as a micromechanical sensor part with a dielectric membrane, as has been disclosed, for example, by DE 43 38 891 A1 and U.S. Pat. No. 5,452,610. The penetration of water into the intake line, for example due to travel on a road wet with rain, can result in contamination of the measuring element. Natural amounts of dissolved salts contained in this splashed water then cause a measurement characteristic curve deviation due to the buildup of salt encrustation on the membrane of the sensor part.
DE 197 35 664 A1 discloses a device in which the measuring element is disposed inside a tubular body that the medium flows through, in which an upstream end of the tubular body extends into a filter chamber and has inlet openings there on a circumference surface in order to reduce the action of dirt particles or water droplets on the measuring element. Particularly with severely contaminated air and a high water content in the intake air of the internal combustion engine, there is the danger that the air filter will become laden with water which then penetrates the filter mat and thereby carries dirt particles along with it. On the downstream side of the air filter, the actually clean side, there is now the danger that the intake air will once again carry along dirt particles and water droplets from the filter surface which will then be undesirably deposited on the measuring element and lead to incorrect measurements or to a failure of the measuring element. Through the placement of inlet openings on the circumference surface, the tubular body according to the prior art does in fact reduce the danger of deposits on the measuring element, but this embodiment produces an undesirable pressure drop which leads to a reduction of the measurement sensitivity.
U.S. Pat. No. 5,507,858 has also disclosed using a screen-like perforated plate in a housing which is connected to a line in order to separate out fluid particles from the air or a gas medium flowing in the line. This housing, though, has two outlets, one for the gas or the air and a second for the fluid. A perforated plate or wire mesh that is circulated around in an approximately longitudinal direction, however, also has the property that a more or less favorable through flow perpendicular to the openings of the perforated plate or wire mesh occurs depending on the angle at which it is set. The through flow capacity of the openings is also a function of the degree of turbulence, the speed of the flowing medium, and the surface roughness of the screen used. Thus the air mass sensor positioned downstream of the wire mesh or the perforated plate produces considerable divergences from a reference without a screen in particular speed ranges, i.e. the measurement of the mass of the flowing medium is supplied under certain circumstances with large tolerances from component to component.
DE 196 47 081 A1 describes screens with different screen opening cross sections. These screens, however, are used to achieve a uniform speed profile and not as a protective screen for a measuring element disposed downstream.
The device according to the invention has the advantage over the prior art that a measuring element can be simply protected from fluid and solid particles and consequently, a measurement characteristic curve deviation is prevented by virtue of the fact that a screen surfacexe2x80x94which is disposed in the line upstream of the measuring element, upstream of a measuring body, or upstream of a tubular body containing the measuring element or the measuring body and which constitutes at least one protective screenxe2x80x94influences the medium flowing toward the measuring element, a gas/fluid mixture, in such a way that the fluid particles and solid particles are conveyed toward a tubular wall or a line wall. As a result, the gas also remains in a center of the line or tubular body and deviations in the measurement signal of the measuring element are reduced through conditioning of the flowing medium by virtue of the fact that longitudinal eddies are produced in a flow direction.
An advantageous embodiment of the protective screen is a configuration of one or more cones, wherein the cone tip(s) is/are aligned counter to a main flow direction and the cone(s) is/are disposed symmetrically around a line parallel to the center line of the line itself because as a result, the flowing medium once again flows in the main flow direction after passing through the protective screen. In this connection, it is advantageous that this line passes through a center of the measuring element or an inlet opening of the measuring body.
Another advantageous embodiment of the protective screen is a combination of side screens which enclose an acute angle with one another.
In this connection, it is advantageous if at least one longitudinal axis of a damming region extends parallel to a longitudinal axis of the measuring element and both of these axes intersect a center line of the line because as a result, the flowing medium once again flows in the main flow direction after passing through the protective screen.
In the embodiment of the protective screen with its screen surfaces, it is advantageous to have a center line of the screen openings to extend inclined in relation to the main flow direction because this causes a deflection of the fluid particles and solid particles.
At high flow speeds and a high fluid content, it is advantageous to enlarge the screen surface area by virtue of the fact that at least two protective screens are inserted into the line, where the one protective screen partially protrudes into the downstream end of the other protective screen.
At high flow speeds, it is advantageous for there to be a smaller cone angle or smaller protective screen internal angle; at low flow speeds, it is advantageous for there to be a larger cone angle or protective screen internal angle.
When there are pulsations in the flow, it is advantageous to also dispose a protective screen with an attack edge or attack tip counter to the return flow direction downstream of the measuring element in the line.
The insertion of a tubular body into the line in addition to the protective screen offers further advantages in the reduction of the action of solid particles and fluid on the measuring element.
Notches and triangular wedges in the attack edge of the protective screen are an advantageous modification to stabilize or condition the flowing medium so that a reproducible measurement of the air mass is possible.
Deviations that occur during the measurement of air mass in different flow testing stands are minimized. Jumps in the air mass characteristic curve are sharply reduced.
In this connection, it is advantageous to dispose the wedges or notches uniformly along the attack edge and upstream, at the level of the measuring element or the inlet opening of the measuring body.